Charged particle source

ABSTRACT

This invention provides a charged particle source, which comprises an emitter and means of generating a magnetic field distribution. The magnetic field distribution is minimum, about zero, or preferred zero at the tip of the emitter, and along the optical axis is maximum away from the tip immediately. In a preferred embodiment, the magnetic field distribution is provided by dual magnetic lens which provides an anti-symmetric magnetic field at the tip, such that magnetic field at the tip is zero.

CLAIM OF PRIORITY

This application claims the benefit of priority of U.S. provisionalapplication No. 62/089,609 entitled to inventors Shuai Li, filed Dec. 9,2014 and entitled “Dual Magnetic Condenser Lens for Electron Gun”, theentire disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a charged particle source for inspection. Theinvention also relates to an electron source with an electron beamcontrol apparatus that can provide higher angular beam current, highspatial resolution, and improvement of throughput for the inspectionprocess of scanning electron microscope. However, it would be recognizedthat the invention has a much broader range of applicability.

BACKGROUND OF THE INVENTION

Undoubtedly, compared to any other technology or knowledge,semiconductor devices not only impact nowadays society but alsoinfluence our daily life. Although it can be traced to two centuriesago, for example Alessandro Volta in 18th century and Michael Faraday in19^(th) century, the history of semiconductor development indeedinfluences mankind in commercial semiconductor devices in 20^(th)century. In the first, vacuum tube transistor is replaced by thesemiconductor devices which mainly include BJT (Bipolar JunctionTransistor) and MOSFET (Metal-Oxide-Semiconductor Field EffectTransistor), and then the semiconductor devices are minimized intointegrated circuits. No matter digital logic circuit device, analogcircuit device or communication devices, these semiconductor devices canbe fabricated on silicon-based substrate or semiconductor compoundsubstrate. Another semiconductor devices are photoelectronic deviceswhich mostly include LED (Light Emitting Diode), LD (LASER Diode) orphotovoltaic cell base on photoelectric effect. Currently, digitalelectronic devices based on MOSFET fabricated in silicon substrate arecommercially the most significant, and the applications of the devicesare processors and memory devices.

Fabrication processes for manufacturing ICs in the silicon substrateinclude cleaning process, oxidation and thermal process,ion-implementation process, thin film deposition, lithography, etchingprocess and CMP (Chemical Mechanical Polishing) process. By thecombination of the above processes, when all electronic devices areformed in the substrate, and then followed by metallization process toelectric connect all electronic devices, a specific application device,such as CPU, ASIC, FPGA, DRAM, or Flash, can be produced. With thetechnology progress of semiconductor process, the smaller width of anelectronic device followed by Moore's law which means transistors aredoubled every 24 months, the more devices in one wafer can be fabricatedto cost down.

The semiconductor fabrication processes include ion implantationprocess, thermal process, thin film deposition process, etching process,CMP (Chemical Mechanical Polishing) process, lithography, and cleaningprocess. And they will be briefed hereinafter.

Ion-implantation process will direct group III or group V atomsimplanted into silicon substrate to alter local electric conductivitysuch that some regions are positive conductivity and some regions arenegative conductivity. Phosphorus or Arsenic atoms are usually used forthe negative conductivity, while Boron atom is usually used for thepositive conductivity.

Thermal process provides formation of thermal oxide layer and annealingfor drive-in after ion-implantation. In the present art, RTP (RapidThermal Process) is popular instead of conventional thermal process infurnace. It includes RTO (Rapid Thermal Oxidation) and RTA (RapidThermal Annealing) to respectively form silicon oxide and repair latticedamages after ion-implantation such that single crystal structure can berecovered and dopant can be activated.

Thin film deposition process includes PVD (Physical Vapor phaseDeposition) and CVD (Chemical Vapor phase Deposition) to form several toseveral tens thin film layers with variant materials and thicknesses onsilicon substrate. Metal layers, formed on a substrate, always provideelectric interconnections among devices, while dielectric layer providesisolation between metal layers. Chemical reactions in vapor phase,happened to form thin films in CVD, include MOCVD (Metal-Organic CVD),APCVD (Atmosphere Pressure CVD), LPCVD (Low Pressure CVD), HPCVD (HybridPhysical CVD), RTCVD (Rapid Thermal CVD), HDPCVD (High Density PlasmaCVD), and PECVD (Plasma Enhanced CVD). Thin films forms by CVD usuallyinclude silicon oxide, silicon nitride, polysilicon, metal tungsten,metal aluminum, and metal titanium nitride. Metal target are heated orbombarded in vacuum such that atoms on the metal target can betransferred to substrate surface to form thin film in PVD, whichincludes evaporation and sputtering. Metal thin films, such as aluminum,titanium, or alloy thereof, are always formed by using PVD. Qualitycontrol of the thin film is critical to IC process, so thin film processmust be monitored throughout the procedure to reflect abnormal, suchthat thickness uniformity and defect and be avoided.

Etching process, which includes wet etch and dry etch, is to removematerial. In the semiconductor process, patterns on a reticle can betransferred to a thin film by using etch process. Wet etching isisotropic by reacting etchant to selective material, and etched profilealways reveals bowl-like shape. Dry etch is popular and anisotropic byreacting plasma in an external electric field with the selectivematerial, and etched profile will reveal vertical-like shape.

CMP is another method to remove material, which introduce slurry betweenpublish pad and wafer with chemical and mechanical reactions to achievewhole wafer planation, such that thin films in the following process canbe formed better. Silicon oxide layer, metal layer and polysilicon layerare most applied in the CMP process.

Lithography process, also named photo-lithography process, is the mostcritical in the semiconductor process, which includes PR (photo Resist)layer coating, soft bake, exposure, development, hard bake, and ashafter etching process. The PR can be selectively removed throughexposure and development, and circuit patterns can be transferred to aspecific material. When the semiconductor process continues shrinking,available RET (Resolution Enhancement technology), such as OPC (OpticalProximity Correction), immersion lithography, and EUV (ExtremeUltraviolet lithography, are applied.

Cleaning process must be processed after all other process recited aboveto avoid uninvited particles or residues to impact device quality, whichincludes rinsing wafer by DI (De-Ionized) water and drying the wafer.Ultrasonic agitation can be applied in the cleaning process. Thisprocess will clean out all pollutions, such as particles, organicmatter, inorganic matter, metal ions.

Defects are inevitably generated in the semiconductor process, whichwill greatly impact device performance, even failure. Device yield isthus impacted and cost is raised. Current defects can be classified intosystematic defects and random defects in general. On the one hand,system defects infer defects will be found repeatedly and systematicallyin wafers, in which defect patterns can be used as reference inclassification to determine root cause of which process incurs suchdefects. In order to increase semiconductor process yield, it iscritical to enhance yield by monitoring, such as by using SEM (ScanningElectron Microscope), systematic defects highly appeared regions in massproduction process to real time eliminate systematic defects. On theother hand, the non-systematic defects, random particle defects, arerandom residues left in wafers. Distributions and characteristicprofiles are important references to distinguish systematic defects fromnon-systematic defects.

More specifically, systematic defects can be classified as reticleerrors in alignment or machine offset, process mistakes incurred byrecipes or materials, prober damages in wafer probing, scratches onwafer surface, and wafer edge effect of topography incurred fromnon-uniformity of PR coating or thermal stress.

The corresponding defects are recited hereinafter in brief. Defectsincurred in lithographic process include PR residue defects due to PRdeteriorated or impurity, peeling defects, bridge defects, bubbledefects, and dummy pattern missing defects due to pattern shift. Defectsincurred in etching process include etching residue defects,over-etching defects and open circuit defect. Defects incurred in CMPprocess include slurry residue defects, dishing defects and erosiondefects due to variant polishing rates, scratched due to polishing.Further, when process nodes continue shrinking, new materials andprocesses will be introduced to inevitably incur new type defects. Forexample, because physical dimension of patterns are smaller than theoptical resolution of the applied lithographic wavelength (193 nm), thecritical dimension exposed on wafers may incur offset. Thinning defectsare another inevitably incurred in the process node shrinking. In orderto reduce RC delay in multi-layered interconnection structures, low-kdielectric layer and cupper material are introduced. Cupper can't beetched and hence damascene process is introduced that metal is filledinto dielectric layer. Therefore, some other hidden defects are coveredunder layer, such as void defects, etching residue defects, over-etchingdefect, under layer particles, and via open incurred in theinterconnection process. Such hidden, crucial defects are too hard to beanalyzed and eliminated.

For the non-systematic defects are mainly random particles defectincurred from particles in air randomly fallen on the wafer, which arenot easy to be identified and resolved.

In order to enhance semiconductor process yield, defects have to beidentified as soon as possible to prevent from impact pouring out.Optical microscope is used in conventional optical inspection whichincludes bright field inspection and dark field inspection. Every die ona wafer is scanned by optical beam and images of every die are generatedand stored. A die-to-die compare is used to identify if there is anyabnormal or defect with locations and images thereof.

When semiconductor nodes continue shrinking, dimensions of defect shrinkalso. Unimportant small defects in previous now become criticaltherefore. It is a challenge to identify such small defects by usingconventional optical inspection tool and a new tool is necessary. Onemethod is to combine the operations of optical inspection and reviewSEM. Because of resolution, the optical inspection is not enough to meetrequirement of identifying defects, but a suspect region in blurredimages can be determined defect-like and reviewed by review SEM withhigh resolution. Thus defects can be identified and analyzed. Anothermethod is to illuminate dual beams on a wafer surface to obtaininterference patterns, and defect regions always have differentinterference pattern to that of the normal region. Thus, defects can beidentified and further analyzed by review SEM. In practice, defects mustbe identified first and locations of the defects are forward to reviewSEM with high resolution to analyze defects.

However, in sub-20 nanometer semiconductor node, optical inspection toolcan't reveal any pattern more, even by using interference method, andhence SEM is the only way to identify defects. Nevertheless, due to thedetected signal electrons in the SEM are secondary electrons, detectionduration inevitably retrogrades significantly compared to that ofoptical inspection. Hence, it is an important issue to fast identifydefects on a wafer by using SEM. An ebeam inspection tool, based on SEM,is currently best solution for defect inspection.

The ebeam inspection tool is to find or identify defects in thesemiconductor process, and relative to review SEM, a large FOV(Field-of-View) and large beam current are commercial means to enhanceinspection throughput. In order to obtain large FOV, a SORIL (SwingObjective Retarding Immersion Lens) system is applied commercially.Moreover, resolution is sometimes lowered, compared to review SEM,enough to capture defects.

The ebeam inspection tool is designed different from the review SEM. Thereview SEM is designed to known, identified defects or suspects ofdefect, so scan duration is long enough to analyze or review defects,and hence it can't process inspection. On the other hand, the ebeaminspection tool, with high scanning rate than the review SEM and highresolution than the optical inspection tool, can identify defects thatthe optical inspection tool in no way to capture.

Furthermore, in lithographic process, some particular patterns may havegreat possibility to incur defects, but won't incur them each time. Thedefects generated by these particular patterns even can't be modifiedthrough recipe tuning or modifying reticle directly. Such a kind ofpatterns is named hot spot, and must be monitored in-line process.

Applications of SEM, except yield management tool of ebeam inspectionand analysis tool of review SEM, may further be metrology tool insemiconductor manufacturing process; that is CD (criticalDimension)-SEM. CD-SEM will measure CD in a wafer with by line-scanningsample with moving stage to reveal process uniformity. Moreover, inorder to obtain exact dimension, resolution is very critical, and thuslow beam current must be applied.

Still another application of SEM is EBDW (E-Beam Direct Writer), ornamed EPL (E-beam Projection Lithography), still based on SEM. Purposesof EBDW are to expose a photoresist directly, and an etching step can beapplied to transfer patterns to a sample after the photoresist isdeveloped. In such a process, there is no reticle necessary, andpatterns are written directly on the sample. Because wavelength of anebeam is superior small than an optical wavelength, finer patterns, suchas nano scale resolution, can be easily obtained.

For the ebeam inspection tool, beam current is always large enough toincrease throughput. Moreover, in VC (Voltage Contrast) mode, defectscan be identified only when enough large beam current is applied.However, large beam current always incurred some issues, such as lowerresolution, larger aberration, and charging. In nowadays shrinkingsemiconductor node, it is necessary to provide a large beam current withhigher resolution.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a charged particle sourcewith large beam and high resolution. A magnetic field distribution isprovided that magnetic field is minimum or zero at the tip of theemitter and, along the optical axis, maximum away from the tipimmediately. The magnetic field distribution can be provided by onemagnetic lens or dual magnetic lens.

Accordingly, the invention provides a charged particle source, whichcomprises an emitter for providing a charged particle beam, and meansfor providing a magnetic field, wherein the magnetic field is minimum ata tip of the emitter, and the magnetic field along an optical axis ofthe charged particle beam reaches maximum immediately away from the tip.

The charged particle beam can be an electron beam, and the chargedparticle source may further comprise an extraction electrode forextracting electrons away from the emitter.

The present invention also provides an electron source, which comprisesan emitter for providing an electron beam along an optical axis, uppermeans for generating a first magnetic field, and lower means forgenerating a second magnetic field, wherein the first magnetic fieldsuperposed by the second magnetic will provide zero magnetic field at atip of the emitter and, along the optical axis, will reach maximumimmediately away from the tip.

The electron source may further comprise an anode for extracting theelectron beam away from the emitter, and may further comprise asuppressing electrode around the emitter. The upper means can be a firstpermanent magnetic attached to the suppressing electrode. The lowermeans can be a second permanent magnetic below the emitter or include anexcitation coil encompassed by a yoke.

The present invention also provides a condenser lens system, whichcomprises a first magnetic lens above an electron source, and a secondmagnetic electron source below the electron source. When a firstmagnetic field generated by the first magnetic lens is anti-symmetric toa second magnetic field generated by said second magnetic lens at a tipof the electron source, a compound magnetic field superposed by thefirst and second magnetic field is weakest at the tip and largestimmediately long an optical axis of the electron source, and thecompound magnetic field provides a high resolution mode. When the firstmagnetic field generated by the first magnetic lens is symmetric to thesecond magnetic field generated by the second magnetic lens at the tipof the electron source, the compound magnetic field superposed by thefirst and second magnetic field is largest at the tip of the electronsource, and the compound magnetic field provides a high throughput mode.

The weakest magnetic field at the tip of the electron source can bezero. The first magnetic lens includes a first excitation coilencompassed by a first yoke, and the second magnetic lens includes asecond excitation coil encompassed by a second yoke. The condenser lenssystem may further comprise a vacuum tube encompassing the electronsource and inside the first and second magnetic lenses.

The present invention also provides an electron beam tool, whichcomprises an electron source for providing an electron beam, thecondenser lens system, an auxiliary lens for condensing the electronbeam, an objective lens for focusing the electron beam on a sample, ascanning unit for scanning the electron beam on the sample, and adetector for receiving signal electrons emanated from the sample.

The electron beam inspection tool may further comprise a plat with aplurality of apertures between the condenser lens system and theauxiliary lens. The objective lens can be an immersion lens, andpreferred SORIL system.

The present invention also provides a method for providing an electronsource, which comprises steps of emitting an electron beam, along anoptical axis, by an emitter, and generating a first magnetic field and asecond magnetic field, wherein a superposed magnetic field by the firstand second magnetic fields has a minimum magnetic field at a tip of theemitter, and, along the optical axis, a maximum away from the tipimmediately.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 is a prior schematic illustration of an immersion type electrongun, wherein an emitter is immersed an magnetic field;

FIG. 2 illustrates that area of virtual source is enlarged due toemitter immersed in the magnetic field;

FIG. 3 illustrates that area of virtual source can be kept small due toelectrons at the tip of the emitter is not immersed in the magneticfield;

FIG. 4(a) to FIG. 4(d) are schematic illustrations of a charged particlesource in accordance with several embodiments of the present invention;

FIG. 5 is a schematic illustration of a charged particle source insidedual magnetic lens with anti-symmetric magnetic field distribution inaccordance with one embodiment of the present invention;

FIG. 6 shows electron optical illustration of electrons emitted from thecathode under the anti-symmetric magnetic field distribution with anembodiment as shown in FIG. 5;

FIG. 7 is a schematic illustration of a charged particle source insidedual magnetic lens with symmetric magnetic field distribution inaccordance with one embodiment of the present invention;

FIG. 8 shows electron optical illustration of electrons emitted from thecathode under the anti-symmetric magnetic field distribution with anembodiment as shown in FIG. 7;

FIG. 9 is a schematic illustration of a charged particle source inside amagnetic lens with magnetic monopole-like field distribution inaccordance with one embodiment of the present invention;

FIG. 10 is a schematic illustration of a charged particle source with avacuum tube configured between the cathode and the dual magnetic lens inaccordance with one embodiment of the present invention; and

FIG. 11 is a schematic illustration of a SEM with the electron source inaccordance with one embodiment of the present invention.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “specimen” generally refers to a wafer or anyother specimen on which defects of interest (DOI) may be located.Although the terms “specimen” and “sample” are used interchangeablyherein, it is to be understood that embodiments described herein withrespect to a wafer may configured and/or used for any other specimen(e.g., a reticle, mask, or photomask).

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide, and indium phosphide.Such substrates may be commonly found and/or processed in semiconductorfabrication facilities.

Turning now to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures is greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to tie samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of anapparatus, column or a device such as a lens”, while “radial” means “ina direction perpendicular to the optical axis”. Usually, the opticalaxis starts from the cathode and ends at specimen. The optical axisalways refers to z-axis in all drawings.

The term, crossover, refers to a point where the electron beam isfocused.

The term, virtual source, means the electron beam emitted from thecathode can be traced back to a “virtual” source.

The present invention relates to a charged particle source, especiallyto an ebeam source which can be applied to SEM, ebeam inspection tool,or EBDW. The ebeam source, in this art, also named e-gun (Electron Gun),is a critical index for generating a high luminance, high throughput,and low aberration. This invention provides an immersion ebeam source bychanging magnetic field distribution.

The ebeam source includes an electron emitter and a lens set. Theelectron emitter or emitter includes a cathode, which is thermionizationtype in early stage. This type emitter constitutes with tungstenfilament, applied by ten thousands voltage to increase temperature toabout 2800K to overcome tungsten work function, such that electrons inthe tungsten filament can escape from the filament to form thermionelectrons. However, due the emission solid angle is very large, radiusof the emitted electron beam and energy spread are very large, andaberration is hence incurred. Moreover, the temperature is too high suchthat raised power consumption, lower vacuum, lower emission rate, anddecayed lifetime are incurred. Furthermore, the long-term hightemperature operation will cause the tip of the filament deformation todeteriorate uniformity of the emitter electron beam. Another material ofcathode, LaB₆, is provided to improve to above mentioned issues, butcost is increased. The field emission electron gun with low aberration,high emission rate, and high reliability is thus provided. Present fieldemission e-gun includes cold field emission type with single crystal Wand Schottky type with ZrO/W, in which high voltage is applied togenerated Schottky effect such that electrons can leave cathode bytunneling effect or overcoming energy barrier. Because the electronswill be emitted at the tip of the emitter, a very fine and bright highcurrent density beam current can be provided. The operation temperature,compared to thermionization type, is low can hence life can be extendedwith lower power consumption. The cold field emission type possessessmall radius of beam current and energy distribution due to tunnelingeffect, while, except small radius of beam current and energydistribution, the Schotty emission type possesses large beam current dueto lower work function. The Schottky emission type is thus nowadaysmainstream.

In order to further lower aberration of the electron source, animmersion type e-gun is developed in which the emitter is immersed in arotational symmetric magnetic field. The magnetic field, non-uniformfield, is provided by a magnetic lens which can be composed ofsolenoidal coil or a permanent magnet. The immersion type e-gun isdifferent to the conventional non-immersion type e-gun, because localelectrons emitted will whirl along spiral trajectories around localmagnetic force lines, instead of optical axis, to converge. Therefore,there will be no crossover formed and Coulomb compulsion effect isreduced. The magnetic lens is a pre-condenser lens to pre-condense theelectron beam from the cathode. The effective angular intensity is henceincreased and the emitted solid angle is reduced. The more electron beamclose to optical axis, the more probe electron beam is increased, andlarger beam current can be provided with less geometric aberration. Thelarger beam current, the more inspection throughput is, because the moreprobe ebeam spot scanned, the more signal electrons detected by thedetector will be. Furthermore, the less time of ebeam staying in apoint, the fast scan rate can be obtained. Such an immersion typeelectron source is applied to ebeam inspection tool based on LVSEM (LowVoltage SEM) in semiconductor manufacturing process.

Detail structure of the immersion type e-gun can be referred to FIG. 1,wherein a cathode 101, surrounded by a suppression electrode 104, emitsan electron beam 110. An extraction electrode 102, below the cathode101, extracting electrons from the cathode 101, and a lens electrode103, below the extraction electrode 102, slightly converges the electronbeam 110. An anode 106, below the lens electrode 103, is grounding toaccelerate the electron beam 110 to a specimen. A magnetic lens 120,co-axial symmetrically surrounding the cathode 101, includes anexcitation coil 124 covered by a yoke 122. The yoke 122, composed ofmagnetic conductive material, has an opening which provides upper polepiece 126 and lower pole piece 128 when the excitation coil 124 excitesto generate magnetic field. The magnetic field distribution, along theoptical axis, has a maximum Bmax at the tip of the cathode, which meansthe emitter 101 is immersion in the magnetic field.

The suppressor electrode 104 will provide a negative field to suppresselectrons from side of the cathode 101 from large angular distributionelectron beam. The electrons emitted from the cathode 101 will spiral inthe immersion magnetic field and is converged to the optical axis. Theextraction electrode 102, lens electrode 103, and the anode 106configure an electrostatic lens to condense the electron beam 110. Theelectron beam 110 thus has a crossover below the anode 106. Thecrossover can be determined by tuning the excitation coil 124 and thelens electrode 103.

Please refer to FIG. 2, detail structure of the cathode 101 of electronsemitted from the cathode 101 is illustrated. The electron beam 210 isemitted from the cathode 201 along the optical axis 200. However, thecathode 201 is immersed in the magnetic field, the maximum field, andthe electrons emitted away from the surface of the cathode 201 willspiral along a local magnetic force line, not the optical axis 200.Please notice that only the electrons emitted at the optical axis 200will spiral along the optical axis 200. The virtual source 216 this willhave larger area. Please refer to FIG. 3, a conventional non-immersiontype emitter is illustrated. The electrons emitted away from the cathode301 will be accelerated to anode far away and will not close to theoptical axis 300 because of no magnetic field. The virtual source 316 inthis situation is less than that of the immersion type e-gun in FIG. 2.The virtual source 216 in FIG. 2 is enlarged, because every emittedelectron in the ebeam 210 will be more converged parallel, or theelectron beam 210 has lowered solid angle. This enlarged virtual source216 through electronic optical devices will be projected on thespecimen, and beam spot size at specimen surface is also enlarged, sothe resolution is thus decreased.

This invention provides a charged particle source wherein the chargedparticle beam emitted away from an emitter is immersed in a maximummagnetic field immediately. Beam current density can be increasedwithout enlarging virtual source, and hence high resolution can beprovided at large beam current. In one embodiment of the invention, thecharged particles are electrons, and can be applied to SEM or ebeaminspection tool.

The magnetic field at the tip of the emitter is the minimum of the fielddistribution, and preferred zero, and, along the optical axis, ismaximum away from the tip immediately, such that most electrons can becondensed to the ebeam or primary beam. Hence, without the magneticfield, the electrons emitted away from the tip will not limit largersolid angle, and the virtual source thus will be smaller than that ofconventional immersion type e-gun. A larger beam current can beavailable also because the magnetic field will converge electrons in theebeam current after the electrons immediately leaves the tip. Therefore,a large beam current with high resolution electron source can beprovided.

This invention also provides a SEM, which comprises an electron source,a column, and a detector. The electron source comprises a cathode foremitting electrons, a suppresser electrode encompassing the cathode forsuppressing emitted electron distribution, an extraction electrode belowthe cathode for extracting electrons from the cathode, an acceleratingelectrode below the extraction electrode for accelerating the ebeam, adual magnetic lens co-axial symmetrical to the optical axis andsymmetric to the emitting plane. The dual magnetic lens will provide amagnetic field distribution that is minimum or zero at the tip of thecathode and along the optical axis largest immediately away from thetip. The column comprises a column aperture below the dual magnetic lensfor selecting beam current, an auxiliary lens below the column aperturefor condensing the ebeam, a detector for receiving signal electrons, anobjective lens for focusing the ebeam on the specimen, a scanning unitfor scanning the ebeam, and a retarding electrode for retarding andfocusing energy of the ebeam. The objective lens, for example MOL(moving objective lens), VAL (varied axis lens), SOL (swing objectivelens), or VAIL (varied axis immersion lens), may scan a large FOV. Avacuum tube may be applied to isolate the dual magnetic lens from thecathode, the suppresser electrode, the extraction electrode, and thecolumn.

The emitted electrons keep high energy till the retard electrode, andare retarded by the retard electrode to a specific landing energy to thespecimen. The landing energy is the voltage difference between specimenand the cathode.

The electron source provided by this invention can be operated at twomodes. The first mode is the large beam current with high resolutionwhen the dual magnetic lens provides an antisymmetric magnetic field atthe tip of the cathode. Due to the anti-symmetric magnetic field at thetip, the magnetic field at the tip is zero and along the optical axis islargest away from the tip immediately. Large beam current and highresolution can be obtained simultaneously.

The second mode is low resolution but largest beam current. The dualmagnetic lens can provide a magnetic field distribution with maximum atthe tip of the cathode, which similar with conventional immersion typeemitter. In this mode, largest beam current is provided and area of thevirtual source is increased. However, for some kind of application, suchas VC mode, defects can be identified at large enough beam current only.Thus, this invention can also provide VC mode inspection.

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

Please refer to FIG. 4(a), which is a first embodiment of thisinvention. This invention provides a dual magnetic lens 420 a and 430 asurrounding a cathode 401 a along an optical axis 400 a, a suppresserelectrode 404 a around the cathode 401 a, and an extraction electrode402 a below the cathode 401 a for extracting electrons from the cathode401 a. The dual magnetic lens 420 a and 430 a may be provided bypermanent magnets, excitation coils, or combination thereof, whichprovides a magnetic field distribution along the optical axis. In thisembodiment, the magnetic field distribution is not symmetric to a plane408 a through the tip of the cathode 401 a. The dual magnetic lens 420 aand 430 a, in this embodiment is not symmetric to the plane 408 a, butcan be symmetric to the planes 408 a. The magnetic lens 430 a may bemovable along the optical axis. The cathode 401 a, being an emitter, canbe thermion type, cold field type, or Schotty type.

The suppresser electrode 404 a provides a negative electric field tosuppress electrons from side of the cathode 401 a from large solid angleebeam distribution. The extraction electrode 402 a assists electronsaway from surface of the cathode 401 a. The magnetic field distribution,in this embodiment, is zero or about to zero at the plane 408 a, andhence the emitted electrons at the tip is not or less influenced by themagnetic field. The virtual source is thus small. And the emittedelectrons, along the optical axis, away from the tip of the cathode 401a, are influenced by the maimun magnetic field Bmax, such that the ebeamis converged along the optical axis 400 a. The magnetic fielddistribution can be adjusted by configuring excitations of the magneticlens 420 a or 430 a, or by moving position of the magnetic lens 430 a.

Please refer to FIG. 4(b), a second embodiment of this invention isprovided, wherein a permanent magnet 420 b is applied to replace themagnetic lens 420 a. The low magnetic lens 430 b can provide a magneticfield by a permanent magnet or an excitation coil. The superposedmagnetic field distribution, by the dual magnetic lens 420 b and 430 balong the optical axis, will be zero or about zero at the plane 408 band maximum Bmax immediately away from the tip of the cathode 401 b.

Please refer to FIG. 4(c), a third embodiment of this invention isprovided. It is provided a single magnetic lens 420 c surrounding acathode 401 c along the optical axis 400 c, a suppresser electrode 404 caround the cathode 401 c, and an extraction electrode 402 c below thecathode 401 c. The magnetic lens 420 c provides a single-pole likemagnetic field along the optical axis by a permanent magnet or anexcitation coil. The magnetic field distribution is minimum, about zero,or preferred zero at the plane 408 c. The cathode 401 c, being anemitter, can be thermion type, cold field type, or Schotty type.

Please refer to FIG. 4(d), a fourth embodiment of this invention isprovided. A permanent magnet is applied to the magnetic lens 420 d,which can provide a magnetic field distribution illustrated in the righthand side of the FIG. 4(d). This kind of permanent magnet may beconfigured by two circular magnets sandwiching none magnet layer. Themagnetic field distribution is minimum, about zero, or preferred zero atthe plane 408 d, and maximum Bmax away from the tip of the cathode 401 calong the optical axis 400 d.

Theoretical analysis for motions of the emitted electrons will bediscussed hereinafter. Under the nonrelativistic condition, the emittedelectrons at conserved rotational symmetric electromagnetic field can bedescribed by cylindrical coordinate system (z, r, φ):

$\begin{matrix}{{\frac{\mathbb{d}}{\mathbb{d}r}\left( {{m_{0}r^{2}\overset{.}{\varnothing}} + {erA}} \right)} = 0} & (1)\end{matrix}$

Integrate the formula (1) by time, an angular momentum equation isobtained:m ₀ r ² {dot over (Ø)}+erA−m ₀ r ₀ ²{dot over (Ø)}₀ +er ₀ A ₀ −−eC  (2)

wherein e is the electron charge, m_(o) is static mass of the electron,C is the initial canonical angular momentum, and r₀, A₀, and {dot over(Ø)}₀, are initial position, initial magnetic vector potential, andinitial angular velocity of the electron. The angular velocity andangular momentum can be obtained:

$\begin{matrix}{\overset{.}{\varnothing} = {{- \frac{e}{m_{0}}}\left( {\frac{rA}{r^{2}} + \frac{C}{r^{2}}} \right)}} & (3) \\{C = {{{- \frac{m_{0}}{e}}r_{0}^{2}{\overset{.}{\varnothing}}_{0}} - {r_{0}A_{0}}}} & (4)\end{matrix}$

From formula (4), in conventional immersion type emitter, the initialmagnetic vector potential A₀ is not zero, and the initial canonicalangular momentum is not zero, even that the initial angular velocity{dot over (Ø)}₀ of the electron can be neglected. Hence, one electron atoff-axis will be emitted with the angular velocity {dot over (Ø)}₀ informula (3) around the off-axis magnetic force line, or the localmagnetic force line in solenoidal motion, wherein the off-axis magneticforce line is the initial magnetic vector potential A₀. Further, due tothe maximum magnetic field is at the tip of the emitter, magnetic forceline density is maxima at the tip along the optical axis, such thatelectrons will converge to the optical axis. On the other hand, if thereis no magnetic field at the tip, i.e. non-immersion type emitter,because there is no off-axis magnetic force line, the initial canonicalangular momentum C is zero, and the electrons will be emitted from thecathode radially.

The aberration incurred is discussed hereinafter. Beam spot size can beobtained:

$\begin{matrix}{D = \sqrt{D_{i}^{2} + D_{s}^{2} + D_{c}^{2} + D_{d}^{2} + D_{e}^{2}}} & (5) \\{D_{i} = {M \cdot D_{0}}} & (6) \\{D_{s} = {\frac{1}{4}{C_{s} \cdot \alpha^{3}}}} & (7) \\{D_{c} = {\frac{1}{2}{C_{c} \cdot \alpha \cdot \frac{\Delta\; v}{V_{0}}}}} & (8) \\{D_{d} = {0.61 \cdot \frac{\lambda}{\alpha}}} & (9) \\{{D_{e} \propto \frac{1}{V^{n}}},{0 < n < 1}} & (10)\end{matrix}$

wherein D is the diameter of the beam spot, D_(i) is the geometric imagediameter of source, D_(s) is the diameter of spherical aberration, D_(c)is chromatic aberration, D_(d) is the diameter of the diffraction ondisc aberration, and D_(e) is aberration diameter by Coulomb effect.And, D₀ is the diameter of the virtual source, M is the magnification,C_(s) and C_(c) are the coefficients of spherical aberration andchromatic aberration respectively, V₀ and Δv are landing energy andenergy distribution respectively, α is solid angle of the ebeam on thespecimen, λ is the De Broglie wavelength of the electron, and V is theenergy difference from the tip to the specimen.

From the formula (5) and (6), diameter of the virtual source D₀ willimpact resolution; that is larger area of the virtual source will incurlower resolution. Hence in this invention an immersion-like electronsource can be provided with high resolution by keeping area of thevirtual source small. Aberration is thus improved at large beam current.

Please refer to FIG. 5, a fifth embodiment of the invention is provided.This embodiment provides the first operation mode; that is highresolution at large beam current. Equal but opposite directionalexcitation currents are provided to the excitation coils 524 and 534respectively in the dual magnetic lens 520 and 530. Thus, along theoptical axis 500, an anti-symmetric magnetic field distribution at theplane 508 is thus obtained, and this anti-symmetric magnetic fielddistribution is the superposed magnetic field by the magnetic fieldsprovided by the dual magnetic lens 520 and 530. The magnetic field iszero at the plane 508 along the optical axis 500, and maximum Bmax awayfrom the tip of the cathode 501 immediately.

FIG. 6 illustrates motions of electrons influenced under electron opticsin this invention at the first mode. The magnetic fields 620 and 630 aregenerated by the dual magnetic lens 520 and 530 respectively, and aresuperposed to the magnetic field distribution 640. Because the magneticfields 620 and 630 are anti-symmetric to the tip of the cathode 601, themagnetic field distribution 640 is zero at the tip and maximum away fromthe tip immediately along the optical axis 600. The electrons thusextracted by the extraction electrode 602 are emitted from the tip ofthe cathode 601 without any magnetic force, and hence a large solidangle can be obtained to reduce area of virtual source. And theelectrons away from the tip immediately will suffer a maximum magneticfield, and converged to the optical axis 600 to form a large beamcurrent. The magnetic field distribution 640 is a round lens with smallspherical aberration. When the electrons 612 with large horizontalvelocity will be blocked by the extraction electrode 602. The energydistribution of the ebeam is limited to lower the chromatic aberration.

Please refer to FIG. 7, a sixth embodiment of this invention isprovided. This embodiment provides a second operation mode; that is lowresolution and largest beam current. Equal and same directionalexcitation currents are provided to the excitation coils 724 and 734respectively in the dual magnetic lens 720 and 730. Thus, along theoptical axis 700, a magnetic field distribution at the plane 708 islargest, and this magnetic field distribution is the superposed magneticfield by the magnetic fields provided by the dual magnetic lens 720 and730. The magnetic field is maxima at the plane 708 and symmetric to theplane 708 along the optical axis 500. The cathode 701 in this embodimentis completely immersed to the magnetic field distribution.

FIG. 8 illustrates motions of electrons influenced under electron opticsin the invention at the second mode. The magnetic field distribution 842is simply superposed to have a largest magnetic field at the tip of thecathode 801. In this embodiment, all electrons emitted from the tip willbe converged and the largest beam current can be provided similar toconventional immersion type emitter. If largest beam current isconsidered only, this invention can also provide this operation mode.

Please refer to FIG. 9, wherein a seventh embodiment of this inventionis provided. The magnetic lens 920 comprises an excitation coil 924which is enclosed by a yoke. The yoke has an opening constitutes theupper pole piece 926 and lower pole piece 928 for generating magneticforce line when the excitation coil 924 is excited by a current. In thisembodiment, the lower pole piece 928 is far away from the tip of thecathode 901, while the upper pole piece 926 is close to the tip of thecathode 901. The upper pole piece 926 thus provides a magneticmonopole-like field near the tip of the cathode 901. The magneticmonopole-like field distribution along the optical axis 900, illustratedat the right hind side of the FIG. 9, has a minimum at the tip of thecathode 901 and maximum immediately away from the tip along the opticalaxis 900.

Please refer to FIG. 10, wherein an eighth embodiment of this inventionis provided. A vacuum tube 1009 is configured to enclose the cathode1001, the suppresser electrode 1006, and the extraction electrode 1002,such that high vacuum can be provided at the tip of the cathode 1001.The vacuum tube 1009 is composed of non-magnetic conductive material,and the magnetic field distribution provided by the dual magnetic lens1020 and 1030 located outside of the vacuum tube 1009 may penetratethrough the vacuum tube 1009.

Please refer to FIG. 11, wherein a ninth embodiment of this invention isprovided. This embodiment provides a SEM with the electron sourcementioned above and a column. The electron source includes a cathode1101 for emitting electrons, a suppresser electrode 1104 around thecathode 1101 for suppressing electrons emitted from side of the cathode1101, an extraction electrode 1102 below the cathode 1101 for extractingelectrons from the cathode 1101, and a dual magnetic lens 1120 and 1130for providing magnetic field distribution along the optical axis 1100.The column includes an beam current plate 1150 below the dual magneticlens 1120 and 1130 for adjusting beam current, an auxiliary lens 1160below the beam current plate 1150 for adjusting image half angle on thespecimen 10, an objective lens 1170 above the specimen 10 for focusingthe ebeam on the specimen 10, a deflection unit 1180 and 1182 inside theobjective lens 1170 for scanning the ebeam, a retard electrode 1184between the objective lens 1170 and the specimen 10 for providing aretarding force to the ebeam, and a detector 1190 for receiving signalelectrons emanated from the specimen.

The beam current plate 1150 may have several apertures for trimming beamto obtain a selective beam current. The auxiliary lens 1160 can varydifferent image half angle at the same beam current. The auxiliary lens1160 is also a magnetic lens provided by the excitation coil 1164 andencompassed by the yoke with an opening. The objective lens 1170, inthis invention, prefers a system which can scan large FOV. In oneembodiment, SORIL (swing object retarding immersion lens) is preferred.The detector 1190, in this embodiment, is located above the objectivelens 1170, but can be located at other positions. The signal electronsmay be secondary electrons or backscatter electrons.

The dual magnetic lens 1120 and 1130 may be operated at two modes. Whenthe electron source provides large beam current with high resolution,the SEM in FIG. 11 can operate high resolution inspection. When theelectron source provides the largest beam current with low resolution,it can operate VC inspection to identify defects.

One prior art, provided by Kenich Saito et al. and titled to “A newdesign for a field emission electron gun immersed in a magnetic-lensfield”, recites an immersion type e-gun, which includes a magnetic lensgenerating a magnetic field immersing the tip of an emitter, or thecathode. The emitter is located near the lower pole piece and immersedmost magnetic fields.

Another prior art, provided by Frosien et al. and entitled to U.S. Pat.No. 5,895,919, recites an emitter, an extraction electrode and anaccelerating electrode immersed in a magnetic field.

Another prior art, provided by Miyoshi et al. and entitled to U.S. Pat.No. 5,371,371, recites an immersion type emitter with a movable magneticlens.

Another prior art, provided by Veneklasen et al. and entitled to U.S.Pat. No. 6,392,333, recites an immersed magnetic lens with a permanentmagnet fastened to a suppresser electrode. Another prior art, providedby Yasuda et al. and entitled to US publication number 2010/0,019,648,recites an e-gun and lithographic tool, wherein an immersion typeemitter is provided.

Another prior art, provided by Jiang et al. and entitled to U.S. Pat.No. 7,821,187, recites an immersion type emitter which can providevariant beam currents and variant resolutions.

Compared to this invention, the prior arts mentioned above onlyconfigures one magnetic lens to immerse the emitter, and lowerresolution is incurred.

Still another prior art, provided by Zhang et al. and entitled to U.S.Pat. No. 7,893,406, recites an immersion type emitter by dual magneticlens, in which one is an immersion lens for pre-condensing and the otheris non-immersion lens for condensing ebeam. Beam spot size and beamcurrent can be adjusted more flexible. Compared to this invention, theemitter is still immersed in the magnetic field with lower resolution.

In summary, the present invention provides a charged particle sourcewhich can be operated at two modes. The charged particle source includesan emitter for emitting charged particle beam, and means for generatinga magnetic field distribution which is minimum or zero at the tip of theemitter and maximum immediately away from the tip along the opticalaxis. The magnetic field distribution, in a preferred embodiment, can beprovided by dual magnetic lens. When the dual magnetic lens providesanti-symmetric magnetic field at the tip of the emitter, a large beamcurrent with high resolution can be provided. When the dual magneticlens provides symmetric magnetic field at the tip of the emitter, alargest beam current, as in the prior art, can be provided also for somespecific applications.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A charged particle source, comprising: anemitter, providing a charged particle beam; an extraction electrode,extracting charged particles away from the emitter; and a means forproviding a magnetic field having a magnetic field strength, wherein themagnetic field strength is zero at a tip of the emitter, and themagnetic field strength increases to a local maximum after the tip andbefore the extraction electrode along an optical axis of the chargedparticle beam.
 2. The charged particle source according to claim 1,wherein said charged particle source is an electron source.
 3. Thecharged particle source according to claim 1, wherein the extractionelectrode is disposed below a top surface of a lower magnetic lens ofthe means for providing the magnetic field.
 4. A charged particlesource, comprising: a charged particle emitter; an extraction electrode,extracting a charged particle beam from a tip of the charged particleemitter; and a magnetic field generator, positioned to provide amagnetic field having a magnetic field strength, wherein said magneticfield strength is zero at said tip and said magnetic field strength is alocal maximum before the extraction electrode along an optical axis ofthe charged particle beam.
 5. The charged particle source of claim 4,wherein said charged particle emitter is a cathode electron source. 6.The charged particle source of claim 4, further comprising a suppressorelectrode disposed radially around said charged particle emitter andcoaxial with said charged particle beam, wherein said suppressorelectrode and said charged particle beam have a first electric polarity.7. The charged particle source according to claim 4, wherein theextraction electrode is disposed below a top surface of a magnetic lensof the means for providing the magnetic field.